test(enum): add test for for #5020: silent conversion to float

Enums (including enum class) are silently converted to floats during
method calls. This can cause overload resolution to fail.

I believe this behavior is a bug. This commit adds tests for that
behavior. These tests fail right now, but they should pass once the
behavior is fixed.

See: https://github.com/pybind/pybind11/issues/5020

tests/test_enum.cpp

@@ -55,6 +55,16 @@ TEST_SUBMODULE(enums, m) {
     m.def("test_enum_to_uint", [](uint32_t) {});
     m.def("test_enum_to_long_long", [](long long) {});
 
+    // Trying to pass an enum to a function that accepts a float should
+    // trigger a type error
+    m.def("test_enum_to_float", [](double) {});
+
+    // When performing overload resolution, calling f(0, ScopedEnum.TWO)
+    // should select f(float, ScopedEnum), NOT f(float, float)
+    m.def("test_enum_overload_resolution", [](double, double) { return "f(float, float)"; });
+    m.def("test_enum_overload_resolution",
+          [](double, ScopedEnum) { return "f(float, ScopedEnum)"; });
+
     // test_duplicate_enum_name
     enum SimpleEnum { ONE, TWO, THREE };
 

tests/test_enum.py

@@ -229,6 +229,28 @@ def test_enum_to_int():
     m.test_enum_to_long_long(m.ScopedBoolEnum.TRUE)
 
 
+def test_enum_overload_resolution():
+    """When performing overload resolution, enums should not be silently
+    converted to floats"""
+    assert m.test_enum_overload_resolution(0.0, 0.0) == "f(float, float)"
+    assert m.test_enum_overload_resolution(0, 0) == "f(float, float)"
+    assert (
+        m.test_enum_overload_resolution(0.0, m.ScopedEnum.Two) == "f(float, ScopedEnum)"
+    )
+    assert (
+        m.test_enum_overload_resolution(0, m.ScopedEnum.Two) == "f(float, ScopedEnum)"
+    )
+
+
+def test_enum_to_float():
+    """Passing an enum to a function taking a float should trigger a type error"""
+    with pytest.raises(TypeError) as execinfo:
+        m.test_enum_to_float(m.ScopedBoolEnum.TRUE)
+    assert str(execinfo.value).startswith(
+        "TypeError: test_enum_to_float(): incompatible function arguments."
+    )
+
+
 def test_duplicate_enum_name():
     with pytest.raises(ValueError) as excinfo:
         m.register_bad_enum()